With the demand for increasing the power and efficiency of electronic devices, heat density on the devices increase rapidly, resulting in the increased possibility of damaging the devices. Therefore, heat removal from the functional devices has become critical in industry.
For example, in the solar industry, the operational temperature of photovoltaic (PV) modules is the key factor in determining the transfer efficiency of the devices. For every 1° C. above 25° C., the electrical output drops by about 0.4% to 0.5%. A typical rooftop PV array may operate at about 55° C. to 75° C., which means electrical output may be about 12% to 25% below the nameplate rating.
In another example, light emitting diodes (LEDs) have drawn increased interest as a potential replacement for the conventional light bulb. However, like PV modules, LED performance suffers with increased operational temperature. For instance, high junction temperatures can cause losses in efficacy, shortened LED life-time and color degradation. This can be problematic as typically about 75% to 85% of the energy used to drive a LED is converted to heat. Under constant operation at routine ambient conditions, the junction temperature in a LED may be 60° C. or greater, which means that the LED's light output can be 10% to 50% below the device's rating.
Three common mechanisms (conduction, convection and radiation) are useful for removing heat from devices. In the case of conduction, energy is carried by the atomic lattice through electrons or through phonon-phonon interactions in solids. Thermal paste and thermal pads operate based on this mechanism. In the case of convection, generally a hot device is contacted with a flow of cold liquid or gas. Heat transfers from the hot device to the cold liquid or gas, which is carried away and replaced with more cold liquid or gas. These systems often require a dedicated system to control the flow of the cooling liquid or gas, and potentially require additional components such as heat sinks or fans. Use of such systems is clumsy and expensive. Further, when the area of the high temperature surface is large, it is practically impossible to use convection for effective cooling. Finally, in the case of thermal radiation, heat is dissipated through electromagnetic wave or photon irradiation without the need of a medium to transfer heat away from the surface. Thermal radiation mechanisms are typically used for high-temperature application, e.g., at 750° C. or above. For example, high emissivity coatings have been reported for use as furnace internal coatings.